The present invention involves an apparatus for holding a device in abutting relation with a workpiece, especially a bracket for holding a thermocouple against a quartz tube in a semi conductor diffusion furnace used in manufacturing semiconductor devices.
Past thermocouple mounting systems allow the thermocouple to fall out or to lose intimate contact with the device for which temperature is to be monitored: e.g., the quartz tube in a semiconductor diffusion furnace. Several approaches have been attempted: clamps, ceramic blocks, and other apparatus. It is important in all of these holding devices that the thermocouple stay in contact with the quartz tube. Previous holding devices for thermocouples have had a tendency to work loose either because of vibration (e.g., from vacuum pumps associated with the system) or because of thermal cycling. Repeated expansion and contraction as the apparatus is heated and cooled during normal operations causes the thermocouple to work loose from its intimate contact with the quartz tube. The latter reason (i.e., thermal cycling) is the most common cause of such loosening of thermocouples.
In the environment in which the preferred embodiment of the present invention is used, the quartz tube is located inside a diffusion furnace, and it is within the quartz tube itself that the semiconductor materials are thermally treated. Quartz tubes are changed periodically; they comprise a liner inside the furnace which is surrounded by the furnace housing and heating coils. The frequency with which quartz tubes are changed depends upon the process in which they are employed, but such quartz tube changing may occur as often as every 40 process cycles (i.e., approximately every 10 days).
When a new or cleaned quartz tube is inserted, the control system must be set up to sense temperature of the quartz tube at predetermined locations and upon the insertion of the new quartz tube into the furnace, the control system must reestablish a profile for proper processing of the semiconductor devices. That is, the control system must heat the furnace and the installed quartz tube, stabilize the system (the furnace and tube), measure the thermocouple's temperatures against the outside of the quartz tube, measure the temperature inside the chamber, and establish temperature offsets to use during processing. This profiling process generally takes 5 to 6 hours.
Presently available (prior art) thermocouple hold downs, with their propensity to fail to keep the thermocouples in place against the outside of the quartz tube, are a large part of the reason for control systems failing to properly profile a system when a new quartz tube is installed. Operators using such improperly profiled systems and apparatus will find that a thermocouple is, for example, running too hot or too cold to be properly employed to accurately measure temperature at a predetermined point. In such cases, repositioning the thermocouple against the quartz tube is required, and profiling must be reestablished. Thus, an additional 5 to 6 hours of processing time is lost in reestablishing profiling.
Further, the only method operators have to detect and determine thermocouple displacement during operation is product quality degradation. Loose thermocouples result in degraded process control which, further, results in poor or degraded product.
Another shortcoming of other attempts to overcome the problems of thermocouple hold downs is that previous attempts, because of too great of mass and too large of contract area, have degraded thermocouple response.
There is, therefore, a need for an apparatus for holding a device in an abutting relation, especially for holding a thermocouple in an abutting relation with a quartz tube in a semiconductor diffusion furnace, which will firmly hold the thermocouple in place despite vibration or thermal cycling which may occur.
It is a further requirement that the apparatus for holding a thermocouple in such an abutting relation have sufficient high temperature properties to withstand the rigors of the environment in which it is employed to be able to withstand heat without melting or otherwise deforming. This is an important consideration since thermocouples may measure up to about 1,200.degree. C.